format table of CDC 9720-850

[Steve Jay]

In v8n181, Jesse Lee (jesse at tcgould.tn.cornell.edu) asks about
the format table for a CDC Sabre 850.

We have a pair of Sabre 850/xy753 systems.  We formatted them under OS 3.5
a long time ago, before we had the new OS 4 format program, with all the
stuff in format.dat.  Under OS 3, we entered the data for the drive
manually, so I'm not positive of the correct entry for format.dat, but the
equivalent information we used was:

	ncyl = 1379, acyl = 2, pcyl = 1381, nhead = 15,
	nsect = 67, bps = 604

I'm not sure about the entries for rpm & bpt, but I think they are correct
in format.dat.  The "sector" switches on the drive have to be set for 604
bytes/sector.  This actually leaves room for 68 sectors/track.  Formatting
for 67 sectors leaves room for 1 "slip" sector per track, so minor media
flaws don't require use of alternate cylinders.  These parameters work out
to 709 MB of space on the drive, which is further reduced to about 668 MB
real space after newfs gets done with it.

Note we use significantly more of the disk (1379 vs. 1358 cylinders, 67
vs. 66 sectors) than Sun's entry for the disk.  I asked Sun why they
appeared to purposely waste space.  The answer was that when you buy a "X
megabyte disk system" from Sun, you may get a CDC drive, a Fuji, or
something else.  The different disks have different capacities.  All of
the drives that might be sold as part of the "X megabyte system" are
described in format.dat so that they have equivalent capacity....they
purposely waste space on bigger disks to make them match the size of
slightly smaller disks from another vendor.  That way, if your disk
breaks, the service person can replace it with a drive from another
vendor, if that's what's available, and not have to worry about the
customer seeing differnt capacitities.  Remember, when you buy from Sun,
you buy an "X megabyte system", not a specific disk drive.  This seems
like a sensible thing for Sun to do.  However, if you buy the drive from
somone other than Sun, you might as well use all of it.

Anyway, we've been running with these parameters for about a year
with no problems.

Steve Jay
Ultra Network Technologies	Domain: shj at ultra.com
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